Internal combustion engines produce many by-products as a result of burning hydrocarbon fuels such as, particulate matter, unburned hydrocarbons (HC), hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), nitric oxide (NO), and nitrogen dioxide (NO2). Although significant advancements have been made in reducing particulate, HC, and CO emissions, there is still a need to develop systems that further reduce the emission of nitrogen oxides (also known as NOx). Devices, such as NOx adsorbers (also known as NOx Traps), and Selective Catalytic Reduction catalysts (SCR) are being developed to accomplish this goal.
NOx adsorbers assist in reducing NOx emissions by storing nitrogen dioxide as nitrates on a catalytic substrate, which contains absorbent materials, during fuel-lean conditions and releasing the nitrates as nitrogen oxides and oxygen during fuel-rich conditions. Once released, the catalytic substrate converts the nitrogen oxides and oxygen into nitrogen (N2) and water (H2O).
NOx adsorbers significantly reduce NOx emissions. However, their susceptibility to sulfur poisoning is a major technical problem. Sulfur, which is commonly found in trace amounts in diesel fuel, binds to the nitrate sites on the NOx adsorbers' catalytic substrate in the form of sulfates (SO4). Because sulfates are more stable than nitrates and carbonates, sulfur species are not released during the fuel-rich regeneration process when carbon dioxide and nitrogen oxides are released; i.e., in the regular operation temperature range for NOx adsorbers of 150° C. to 500° C. Accumulating over time, sulfates significantly decrease the ability of NOx adsorber to store and release nitrogen dioxide. This results in an overall decreased efficiency of the NOx abatement system and an increase in NOx emissions.
Maintaining NOx adsorber efficiency enables proper function of NOx abatement systems that employ these devices. Therefore, there is significant need to develop methods that will remediate sulfur poisoning of NOx adsorber devices.